Conductive plastic compositions have been well received as desirable raw materials for fabrication of a variety of specialized accessories and components, including static electricity dissipation devices, electrical heating elements, equipment parts for high frequency protection and/or electro magnetic interference (EMI) shielding, video discs, anti-static packaging profiles, and a variety of other electrical components such as electrodes, terminals, connectors, and the like.
Thermosetting or heat-curable polymer systems have, to date, been the most prominent of such conductive plastic materials which have been developed. For certain electrical applications, the resistance of many thermosetting materials to high temperature service conditions is a major consideration. However, generally a more important factor resides in the inherent reactivity responsible for the thermosetting character of these materials which tends to increase the polymeric interactions with the finely subdivided conductive solids (e.g., metallic powders, carbon blacks and the like) that must be incorporated into polymeric base materials in order to provide the appropriate levels of conductivity.
Most thermoplastic resins, on the other hand, are considerably less likely to react with additions of finely divided solid fillers. This usually results in an actual deterioration of many structurally significant physical properties when thermoplastic resins loaded with carbon blacks, powdered metals and the like to the amounts required for reaching practical levels of electroconductivity. Such deficiencies have severely limited applications for these conductive thermoplastic compositions, confining them, for the most part, to the fabrication of at least partly supported auxiliary elements and secondary components like seals, gaskets, inserts and electrodes.
In spite of such difficulties, filled thermoplastic systems have, of course, continued to receive attention since rigid thermoplastic resins offer definite advantages over most thermosetting materials. These advantages include, for example, ease of handling, melt processing convenience and the simplicity of fabricating finished articles therefrom by well-known high speed plastic forming techniques such as, for example, extrusion and injection molding.
Indicative of approaches which have been taken in an effort to develop carbon black filled thermoplastic compositions with the necessary overall performance and utility are those disclosed in the publications summarized below.
U.S. Pat. No. 4,241,120 to Data et al discloses a method of formulating carbon black filled thermoplastic resin compositions wherein the carbon black particles are modified by grafting polymers onto the carbon black particles prior to adding them to a PVC base molding composition, said compositions containing from 12% to 40% carbon black suitable for video discs having low shrinkage characteristics.
U.S. Pat. No. 4,228,050 to Martin et al discloses a carbon black filled compression molding composition containing from 12% to 40% carbon black suitable for video discs having low shrinkage characteristics.
U.S. Pat. No. 4,151,132 to Khanna describes a carbon black filled molding composition containing 12% to 20% conductive carbon black particles, about 10% vinyl chloride-vinyl acetate copolymer, 10% vinyl chloride-maleate ester copolymer, and about 15% to 17% of polymeric plasticizers and processing aids with about 3.5% of two or more metal stabilizers and 1.5% of three or more lubricants, the remainder being a copolymer of vinyl chloride containing about 6% to 8% propylene.
Additional approaches to solving the problems of providing thermoplastic molding compositions have been, for example, the use of graphite/carbon fibers; the use of special plastic materials such as hydroxyl terminated polyether (HTE) and the like as reviewed, for example, in Modern Plastics, p. 62 (June, 1979).
These various products and/or directions, however, appear to be limited in their application since they do not allow for the high speed, "molten state," mixing and molding operations for which thermoplastics are so well suited and the reason for which thermoplastics are usually selected in commercial practice. Also, it will be appreciated that many of these materials will raise the cost of the manufactured item prohibitively when compared to the commercial alternatives already in use.
In view of the apparent state of this art, a considerable need continues to exist for a flame retardant, thermoplastic molding composition of high electroconductivity. In particular, a clear need is sensed for such compositions which are not only derived from a thermoplastic resinous matrix, but which can also be economically and conveniently prepared and dependably fabricated by conventional high speed techniques into a wide variety of shaped articles having both good conductivity and sound physical integrity. One of the most challenging raw material requirements in this field is the need for conductive thermoplastic molding and extrusion compounds suitable for forming flame retardant structural members of sufficient size, mass and complexity to serve as electronic equipment housings, dampers and/or shields for absorbing or blocking out electromagnetic field effects or other high frequency electrical emissions. Thus, for example, the computer and auto industries have set guidelines which indicate that materials having a shielding effectiveness (SE) of 20 to 30 dB will meet 50% of their needs, while an SE of 30 to 40 dB will meet 95% of their needs. Shielding effectiveness is an absolute ratio normally expressed in decibels (dB) and defined on a logarithmic scale through the following equations:
SE=20 log (Ei/Et) PA1 SE=10 log (Pi/Pt)
or
where E is the field strength in volts per unit length, P is the field strength in watts per unit area, i is the incident field and t is the transmitted field. Alternatively, SE can also be expressed on a linear scale as a percent attenuation (PA). PA is simply (Ei/Et).times.(100) or (Pi/Pt).times.(100). Thus, 99% attenuation corresponds to 20 dB, 99.9% to 30 dB and 99.99% to 40 dB. Finally, it should be pointed out that there is often a crude correlation between the shielding effectiveness and the volume resistivity, such that a volume resistivity of lower than 6 ohm-cm usually ensures that the shielding effectiveness will be at least 30 dB.
It is also understood, however, that this level of shielding effectiveness is not needed for "anti-static" applications and, therefore, lower levels of protection will suffice, for example, volume resistivity levels of less than 1.times.10.sup.8 ohm-cm.
Accordingly, a primary goal of the present invention is the production of a family of flame retardant, thermoplastic molding and extrusion compounds of high electroconductivity which can be readily shaped even by fast thermoplastic processing techniques to form rigid articles having well balanced all around physical properties and adequate structural stability for many diversified electrically conducting specialty applications. A more specific objective of the invention is to formulate flame retardant, thermoplastic molding and extrusion compounds, the ingredients and composition of which are further restricted and optimized so that exceptional levels of electroconductivity as well as outstanding physical properties are obtained in the articles molded therefrom without the need for grafting of the carbon black particles to a polymer. Such optimized molding and extrusion compounds are particularly needed for certain specialized structural uses, such as EMI shielding members, electronic equipment housings, and the like, as well as anti-static materials such as video discs and packaging profiles, and thus represent a preferred embodiment of the present invention.